1. Field of the Invention
The field of the invention is that of digital mobile radio systems. The invention applies in particular to cellular mobile radio systems such as systems conforming to the GSM (Global System for Mobile communications) standard, for example.
More particularly still, the invention concerns the exchange of digital signals, whether they constitute data or speech, in time-division multiple access (TDMA) time-division multiplex systems.
2. Description of the Prior Art
The TDMA technique divides time into frames of fixed and predetermined duration, the frames being in turn divided into time slots. Each call is associated with one or more time slots.
Thus a frame comprises N time slots that can correspond to N calls. Each receiver is able to extract the time slots addressed to it in order to reconstitute the source signal. In this way N calls can be transmitted in the same frequency band.
Where data communications are concerned, mobile radio systems like the GSM system conventionally provide two services, corresponding to two different levels of quality. Thus, for transmission of data, the GSM system provides a first data communication mode called the full rate mode, in which a time slot is transmitted in each frame, and a second transmission mode called the half rate mode, in which the data signal is transmitted in one time slot only every two frames, on average.
In this second mode, the resource allocated to a call is halved compared to the first mode. This frees up resources for other calls.
This halving of the total bit rate of the call naturally requires modification of the channel coding used, to retain the same user bit rate, in other words, the half rate mode corresponds to channel coding with half the yield of that of the full rate mode. In the GSM system, the two modes respectively correspond to raw bit rates of 11.4 kbit/s and 22.8 kbit/s.
Consequently, the efficacy of half rate channel coding is less than that of full rate channel coding. For this reason the half rate mode can be used only when transmission conditions are good and/or the transmission quality required is average, in other words when a relatively high bit error rate can be tolerated. If the transmission channel is subject to interference and/or the data requires a higher transmission quality (i.e. a lower bit error rate) the full rate mode must be used.
According to the GSM standard, a transmission mode is chosen at the time the call is set up and is retained throughout the call. This technique has two drawbacks:
if the service in question requires a transmission quality corresponding to the use of the half rate mode under normal conditions of operability and if the half rate mode is adopted, should the channel then be subjected to a higher level of interference, exceeding the operability limit of the system (set at a C/I value of approximately 9 dB), the call in progress is suddenly cut off; under difficult coverage conditions a value of C/I below 9 dB may be encountered;
if the service in question requires a transmission quality corresponding to the use of the full rate mode under normal conditions of operability and if the full rate mode is adopted, should the channel subsequently be subject to a lower degree of interference, the channel coding employed is of higher quality than is required; the transmission channel is therefore unnecessarily occupied in alternate frames (causing unnecessary interference in neighboring cells).
In mobile radio systems these problems are major problems since the transmission channel changes continually with the movement of the mobile station and the movement and the activity of the sources of interference, etc. As a result, the full rate mode is usually chosen, for safety, and this leads to high and often unnecessary consumption of the transmission resource.
There are also two configurations in the case of speech signals (full rate mode and half rate mode), which correspond to the use of different speech encoders (source coding) and different channel encoders, the two pairs of encoders (source and channel) providing respective raw bit rates of 22.8 kbit/s (full rate) and 11.4 kbit/s (half rate).
Problems similar to those described above for data are also encountered with speech.
A major objective of mobile radio system designers is to limit the quantity of data transmitted, for a number of reasons and in particular:
to increase the number of calls in the multiplex;
to reduce the transmission time (in the case of transmitting data);
To this end, U.S. patent application U.S. Pat. No. 5,327,576 proposes modification of the mode of transmission used, during a call, on the basis of the measured bit error rate.
To be more precise, in the method described in the above application, the base transceiver station (the station managing all calls with mobiles in a given cell) measures the bit error rate of a given call and selects one or other of the transmission modes according to the measured error rate.
This technique improves transmission resource use. It has a number of drawbacks, however, that the novel technical approach of the invention clearly highlights. In particular, it is based on an analysis of the transmission channel as seen from the base transceiver station only (or from the mobile station only), which leads to non-optimum resource use, as will emerge below.
One object of the invention is to overcome these various drawbacks of the prior art.
To be more precise, one object of the invention is to provide a method of adaptation of the air interface (essentially corresponding to layers 1 (physical) and 2 (link) of the ISO model) in a mobile radio system that minimizes the occupancy of transmission channels by reducing the quantity of resource allocated to a call on average and by limiting interference induced by a call in neighboring cells.
This object of limiting interference is crucial in cellular mobile radio systems in particular. In these systems, the same frequency band is allocated to several geographically dispersed cells. Although the distribution of the cells is defined to maximize the distance between them, it is by no means rare for the signals of a given cell to suffer interference from those of other cells using the same band to a degree that is above an acceptable limit for the system.
In a cellular system, a maximum interference level enabling the specified transmission quality to be provided is usually fixed. An object of the invention is therefore to provide a method of the above kind whereby the specified transmission quality continues to be provided if the interference exceeds this maximum level.
An object of the invention is therefore to provide a method of the above kind in which the untimely cutting off of calls is reduced.
In other words, an object of the invention is to expand the range of operability of the system, in particular under difficult transmission conditions.
Another essential and primordial object of the invention is to provide a method of the above kind whereby the number of calls can be increased. In other words, an object of the invention is to reduce, on average, the resource used to transmit a service in order to increase the number of users in the system, i.e. the number of calls per cell.
In one particular embodiment of the invention, a secondary object of the invention is to provide a method of the above kind for transmitting asynchronous data easily and as fast as possible, in particular when it is not possible to free up the same resource in both communication directions.
Another object of the invention is to provide a method whereby the network infrastructure is simplified. In the conventional way, network planning must offer acceptable operability (C/I≈9 dB in the GSM system) everywhere (or over a certain portion of the area of each cell), which in some cases constitutes a very serious constraint.
An object of the invention is therefore to remove this constraint by widening the operability range (beyond 9 dB, for example in the case of the GSM system) to allow more efficient planning by reducing the number of sites.
Another object of the invention is to provide a method of the above kind that is equally applicable to speech signals and to data signals.
These objects, and others that will emerge hereinafter, are achieved in accordance with the invention by means of a method of adaptation of the air interface in a mobile radio system enabling exchange of bidirectional digital signals between at least one mobile station and at least one network entity, such as a base transceiver station, and providing at least two coding modes, each mode corresponding to a predetermined source code and a predetermined channel code for the transmission of a wanted signal for each transmission direction, and each source code corresponding to a given user bit rate and each channel code corresponding to a given coding efficiency and therefore to a given total bit rate for a given source code, wherein, at the time of a call between a mobile station and a base transceiver station, two separate analyses of transmission quality are carried out for each transmission direction, respectively, and, for each transmission direction, one of said coding modes is selected in accordance with the corresponding transmission quality analysis.
The invention is based on a novel approach to the transmission channel, which is treated separately for each transmission direction. Transmission conditions can be very different for the two transmission directions. One reason for this is that the sources of interference are different. In the uplink direction, from the mobile station to the base transceiver station, the sources of interference are essentially the mobile stations moving in the co-channel cells; in the opposite, downlink direction, the sources of interference are neighboring base transceiver stations. The masking effects are therefore different.
The invention therefore optimizes three things:
the analysis of transmission conditions, which were previously viewed from one end only (generally the base transceiver station); it was therefore possible to regard as xe2x80x9cgoodxe2x80x9d a channel that was xe2x80x9cbadxe2x80x9d in the opposite direction, and vice versa; the invention presupposes feedback of quality information (in at least one direction) before a decision is taken;
the selection of the coding mode, which is selective for each transmission direction; and
the encoding mode used, since the invention enables selective action at the source coding and/or channel coding level.
It should be noted that the approach adopted by the invention is in no way obvious in the light of the prior art techniques. Apart from the fact that it is based on a novel approach to the transmission channel, a number of modifications are required in order to put the invention into effect, in particular with regard to the exchange of information (measured quality and/or change of coding mode) between the two stations.
Some mobile radio systems, like the GSM system, allocate resources on a fixed basis, through at least two transmission modes corresponding to different transmission resources allocated to a call.
To be more precise, a transmission mode defines a coding mode/allocated resource combination. The transmission mode therefore corresponds to the use of a certain coding mode and the allocation of a certain resource. A coding mode can therefore correspond to a plurality of transmission modes.
In this case, in accordance with the invention, on the occasion of a call between a mobile station and a base transceiver station, one of said unidirectional transmission modes is advantageously selected for each transmission direction, uplink and downlink, in accordance with at least one of said transmission quality analyses and the quality required for said call, and possibly the traffic load.
In other words, a transmission mode is chosen when a call is initialized, corresponding to a required level of quality, for example. The invention enables the transmission mode to be changed during a call, corresponding to a change of coding mode, as soon as this is possible (changing to a mode that is less robust with respect to transmission errors, but offering a higher level of quality and/or a lower consumption of resources) or necessary (changing to a mode that is more robust with respect to transmission errors, but consuming more resources and/or enabling the service to be maintained at the cost of a slight reduction in quality, subject to constraints of availability, of course).
Various situations can be envisaged, according to whether the mobile radio system in question requires symmetrical allocation of resources for each transmission direction or not.
In the second case, which can be regarded as equivalent to two unidirectional calls, the principle can be very simple: the change of mode in one direction is effected as soon as the change of coding mode to a less robust, respectively more robust mode with respect to transmission errors is possible, respectively necessary. A change of the coding mode is necessary if the quality of the channel deteriorates. It is possible if the quality of the channel improves to the point that transmission quality can be maintained using a coding mode consuming less resource or increased if the resource already allocated is retained.
In the former case, which corresponds to the GSM system, for example, direct application always leads to the choice of identical coding modes for both directions. There is therefore a bidirectional transmission mode that corresponds to the use of the same unidirectional mode in both transmission directions. A change of bidirectional transmission mode is therefore effected if a change of coding mode (compatible with the transmission mode) is possible for both directions or necessary for at least one direction.
Thus, in a system in which said transmission modes correspond to an allocation of identical transmission resources in each of said transmission directions, a change of transmission mode to a transmission mode corresponding to a greater transmission resource is effected if at least one of the coding modes selected in at least one transmission direction corresponds to a total bit rate incompatible with the resource allocated in the current transmission mode and the additional transmission resource required is available (in which case the same coding mode is applied to both transmission directions, this coding mode being that of the two modes selected requiring the greater amount of the resource), and a change of transmission mode to a transmission mode corresponding to a lesser amount of transmission resource is effected if coding modes consuming less resource are selected in both directions. A bidirectional transmission mode is then chosen corresponding to the use of the same coding mode in both directions, the latter mode being that of the two modes selected in each direction consuming the greater amount of the resource.
The same approach can be adopted for selecting between bidirectional transmission modes which are the same in terms of use of resource but different in terms of robustness.
The invention is naturally not limited to two coding modes or to two transmission modes. To the contrary, it is a simple matter to generalize the invention to n coding modes and m unidirectional transmission modes (mxe2x89xa7n, it being possible for the same coding mode to correspond to a plurality of transmission modes). On the basis of n coding modes it is possible to define n bidirectional transmission modes for which the same coding mode is used in both directions.
In accordance with one important feature of the invention, at least one modified transmission mode is also defined, in which the coding modes used in the two directions are different.
Accordingly, in the situation in which there are n different unidirectional coding modes, it is possible to define:
n bidirectional transmission modes (known as primary modes) corresponding to the use of the same coding mode in both directions;
n(nxe2x88x921) bidirectional transmission modes (known as secondary modes) corresponding to coding modes in both directions. Among these n(nxe2x88x921) modes there may exist situations in which the raw bit rate is different and therefore corresponds to different required minimal resources in the two transmission directions (to be more precise, there are modified transmission modes of two types:
modes with the same raw bit rate in both directions, and modes with asymmetric raw bit rates). If the system requires symmetrical resource allocation, the resource corresponding to the highest raw bit rate is allocated. The resource allocated to at least one of said transmission directions is therefore greater than the resource required to transmit the information coded in the corresponding coding mode, and said coded information is divided between a fraction of the time slots corresponding to said allocated resource.
This type of modified transmission mode is entirely novel. It is possible only because the invention provides a different approach to quality for each transmission direction. In the case of a bidirectional mode with asymmetric raw bit rates, it enables time slots to be freed up in one direction even if this is not possible in the other direction. Moreover, the invention is equally specifically concerned with a modified transmission mode of this kind.
Two strategies can be envisaged for time slots that are not used by said call:
either they do not carry any signal, which reduces interference with neighboring cells; if there is no transmission in a cell, the latter does not cause any interference in its neighbors,
or they are allocated to the transmission of asynchronous data.
In one particular embodiment of the invention (intended in particular for adaptation of the current GSM standard for data communications), said transmission modes comprise:
a first (full rate) mode in which said data is transmitted at the rate of one time slot every signal frame, and
a second (half rate) mode in which data is transmitted at the rate of one time slot every two signal frames.
In this case, a modified transmission mode advantageously consists in:
transmitting information coded in a first coding mode at the rate of one time slot every signal frame (full rate) in a first transmission direction, and
transmitting information coded in a second coding mode at the rate of one time slot every two signal frames (half rate) in a second transmission direction,
the resources allocated to the call in both transmission directions corresponding to the resource needed to transmit data in said first mode.
The analysis of transmission quality advantageously consists in determining at least one of the following:
the bit error rate (BER) of the received signal,
the power of the received signal,
the distance between the mobile station and the base transceiver station,
an estimate of the impulse response of the transmission channel,
the time alignment,
the signal to noise ratio,
the signal to interference ratio (C/I).
In one advantageous embodiment of the invention said selection of a coding mode allows additionally for at least one of the following:
a required level of quality for the call in progress,
a required level of quality for at least one transmission direction and for the call in progress,
a type of service conveyed by said call,
the traffic load.
The selection of a coding mode preferably includes a step of comparing information representative of the transmission quality with at least one predetermined threshold, to be more precise with the same number of thresholds as coding modes.
Said quality information is advantageously compared with different thresholds according to the level of quality required for the call in progress, if there is more than one level of quality.
It is advantageous to define two sets each of at least one threshold, a first set being used when the measured transmission quality deteriorates and a second set being used when the measured transmission quality improves.
This avoids incessant changing of modes when the measured level is near a threshold (this is known as the xe2x80x9cping-pongxe2x80x9d effect).
Said thresholds are preferably predetermined values of the signal to interference ratio (C/I).
In a preferred embodiment the decision to change coding mode and/or transmission mode is taken in said base transceiver station, said mobile station transmitting to said base transceiver station information representative of transmission quality in the base transceiver station to mobile station direction.
More generally, the method of the invention preferably includes a step of selecting between at least two source codes and/or a step of selecting between at least two channel codes.
Said selection of a coding mode is carried out in such manner as to limit the quantity of resource allocated in each transmission direction and/or to optimize transmission quality.
For example, a source code and a channel code may be chosen to maintain the current raw bit rate as far as possible, and therefore to offer the best possible transmission quality without modification of the resource, or to provide the best possible transmission quality subject to modification of the resource.
The invention also concerns a base transceiver station of a mobile radio system implementing the method as defined above. A base transceiver station of this kind advantageously comprises:
means for determining at least a first indication representative of transmission quality in the mobile station to base transceiver station direction,
means for receiving a second indication representative of transmission quality in the base transceiver station to mobile station direction,
means for modifying the coding mode and/or the transmission mode in each transmission direction in accordance with said first and second indications, and
means for transmitting to said mobile station information representative of the coding and/or transmission modes selected.
It also concerns a corresponding mobile station including:
means for determining at least one indication representative of transmission quality in the base transceiver station to mobile station direction,
means for transmitting said indication to said base transceiver station, and
means for receiving an indication representative of the coding and/or transmission modes selected.
Other features and advantages of the invention will emerge from a reading of the following description of a preferred embodiment of the invention given by way of non-limiting illustrative example and from the accompanying drawings.